A diazonium salt has high chemical activity and is readily able to react with a phenol derivative or a compound having an active methylene group, called a coupler, to form an azo dye. The diazonium salt is also light-sensitive and, as a result, decomposes on exposure to light to lose its reactivity with coupler. Due to such properties, a diazonium salt has long been used in light-sensitive recording media exemplified by diazo type paper (see Nihon Shashin Gakkai (ed.), Shashin Kogaku no Kiso "Higin-en Shashin hen", pp. 89-117 and 182-201, Corona K. K. (1982)). In recent years, the ability of the diazonium salt to decompose on exposure to light and lose its activity has been utilized in recording materials which fix an image. For example, a light-fixable heat-sensitive recording material has been proposed, in which a diazonium salt and a coupler are thermally reacted to form an image, which can be fixed by exposure to light (see Koji Sato, et al., Gazo Denshi Gakkaishi, Vol. 11, No. 4, pp. 290-296 (1982)). However, these recording media using a diazonium salt as a coloring factor are disadvantageous in that the diazonium salt gradually undergoes thermal decomposition even in a dark place due to its high activity. As a result the shelf life of these recording materials is short.
Various approaches have been proposed to overcome the instability of the diazonium salt. One of the most effective means is to microencapsulate the diazonium compound. A microencapsulated diazonium compound is isolated from the factors which may accelerate decomposition of the diazonium compound, such as water and a base, and remarkably suppresses the decomposition, thereby providing a recording material having a prolonged shelf life (see Tomomasa Usami, et al., Shashin Gakkaishi, Vol. 26, No. 2, pp. 115-125 (1987).
A diazonium salt is generally microencapsulated by a process comprising dissolving a diazonium salt in a hydrophobic solvent to form an oily phase, emulsifying the oily phase in an aqueous solution of a water-soluble high polymer (aqueous phase) by means of a homogenizer, etc. in the presence of a microcapsule wall-forming monomer or prepolymer added to either one or both of the oily phase and the aqueous phase, whereby a polymerization reaction takes place or a polymer is precipitated on the interface between the oily phase and the aqueous phase to form a polymeric microcapsule wall. For greater detail, refer to such literature as Choshi Kondo, Microcapsule, The Nikken Kogyo Shimbun Ltd. (1970) and Tamotsu Kondo, et al., Microcapsule, Sankyo Shuppan K. K. (1977). The thus formed microcapsule wall comprises crosslinked gelatin, alginates, cellulose derivatives, urea resins, urethane resins, melamine resins, nylon resins, etc. Of these microcapsule wall materials, those having a glass transition temperature slightly higher than room temperature, such as urea resins or urethane resins, have non-permeability at room temperature and turn permeable at temperatures higher than the glass transition temperature. Microcapsules having such a microcapsule wall, called heat-sensitive wall microcapsules, are useful for heat-sensitive recording materials. That is, a recording material comprising a support coated with heat-sensitive wall microcapsules containing a diazonium salt, a coupler, and a base retains the diazonium salt in a stable state for an extended period of time and easily develops a color image on heating, which color image can be fixed by exposure to light.
Aside from improving stability of a diazonium compound by microencapsulation, the inventors studied ways to obtain improvements in aging stability and oil-solubility of a diazonium compound and found that a 4-substituted amino-2-alkoxybenzenediazonium salt exhibits excellent performance properties as disclosed in JP-A-4-59288 (the term "JP-A", as used herein, means an unexamined published Japanese patent application). As a result of further studies using a combination of this specific diazonium compound with a coupler, the present inventors found that a combination using a barbituric acid derivative as the coupler develops a red dye having a satisfactory hue as disclosed in JP-A-4-197782. However, a recording material containing the 4-substituted amino-2-alkoxybenzenediazonium salt is still unsatisfactory in both raw stock aging stability (resistance to coloring of the background before recording) and record stability (light fastness of the color image). In order to solve these problems, the present inventors have conducted extensive investigations and, as a result, found that a combination of a diazo compound having a specific substituent and a barbituric acid derivative (as the coupler) having a specific substituent as a coupler exhibits excellent performance properties. The present invention has been completed based on this finding.